The present invention relates to air data systems that provide accurate compensation of sideslip of an air vehicle utilizing independent probes that are not pneumatically coupled, but which have processors for interchanging electrical signals between the probes. These probes are sometimes referred to as multi-function probes (MFPs). One type of MFP is the SmartProbe(trademark) sold by B. F. Goodrich Company. Multi-function probes include processing circuitry located at the probe itself as part of its instrument package. During sideslip of the air vehicle, compensation of various local (to the probes) parameters or signals, such as angle of attack and static pressure, is necessary for accurate determination of aircraft angle of attack and other aircraft parameters including determination of altitude from static pressure or other means. This requirement for accuracy in altitude indications is particularly important in Reduced Vertical Separation Minimum (RVSM) space areas of the air traffic control system.
In conventional air data systems, symmetrically located probes on opposite sides of an aircraft can be pneumatically connected so that the pressure signals are averaged between the right side of the aircraft and the left side of the aircraft. This average provides a static pressure that closely approximates the necessary correction for side slip effects. In most conventional systems (pneumatically averaged systems), although corrections are made for Mach number and aircraft angle of attack, additional corrections for side slip are not done since it is assumed that the pneumatic average of local static pressure and the average of local angle of attack accomplishes this task. It is rare that this averaging technique introduces enough error to warrant additional corrections for side slip.
However, MFPs are connected only electrically in order to eliminate the need for pneumatic tubing passing between the probes on opposite sides of the aircraft or between probes on the same side of the aircraft. This means that each probe is pneumatically independent, even if it is electrically communicating with other probes. In RVSM airspace, there is a need for dual redundant systems for static pressure estimation. While information can easily be exchanged between the processing circuitry of different probes, the need for determining sideslip effect remains. In the case of symmetrically located MFPs on opposite sides of the aircraft it is possible to accomplish the same side slip compensation, as done in the traditional pneumatic systems, by averaging the pressures and angles of attack electronically. Computational fluid dynamic analysis has shown that position errors on an individual probe can be up to 600 feet per degree of sideslip in typical RVSM airspace flight conditions, for example, 41,000 feet, Mach 0.8, and a sideslip angle of 2 degrees. It is thus apparent that the sideslip effect must be corrected to obtain the necessary accuracy for certification by aviation authorities.
One possible method of determining aircraft sideslip is to utilize inertial input data from an inertial reference unit (IRU) or other inertial navigation systems. However, it has not historically been known how to implement an accurate electronic correction to air data parameters for aircraft sideslip using inertial inputs. This is due to a lack of reliable in-flight data and the necessary algorithms to incorporate inertial rates and accelerations.
The present invention relates to multi-function air data sensing systems which provide redundancy in the correction, calculation, and presentation of various air data parameters, such as aircraft angle of attack, static pressure, pressure altitude, Mach number, and indicated airspeed. Aerodynamic sideslip angle is a measure of the magnitude of a cross component of airspeed to the forward component of airspeed.
A method of the invention allows the determination of aircraft sideslip using an air data probe and an inertial reference unit. One component of aircraft sideslip angle can be attributed to the lateral accelerations and forces on the aircraft. This component will be labeled xcex2L. A second component of aircraft sideslip angle can be attributed to the angular rates of the aircraft""s motion. This component will be labeled xcex2A. The lateral sideslip component (xcex2L) and the angular sideslip component (xcex2A) are combined to obtain a total sideslip angle xcex2TOTAL for the aircraft. The total sideslip angle xcex2TOTAL can be used to compensate static pressure, angle of attack and other aircraft parameters for sideslip effects.